Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharging rate have been commercialized and widely used.
Lithium transition metal composite oxides have been used as a positive electrode active material of the lithium secondary battery, and, among these oxides, a lithium cobalt composite oxide of LiCoO2 having a high operating voltage and excellent capacity characteristics has been mainly used. However, since the LiCoO2 has very poor thermal properties due to an unstable crystal structure caused by lithium deintercalation and is expensive, there is a limitation in using a large amount of the LiCoO2 as a power source for applications such as electric vehicles.
Lithium manganese composite oxides (LiMnO2 or LiMn2O4), lithium iron phosphate compounds (LiFePO4, etc.), or lithium nickel composite oxides (LiNiO2, etc.) have been developed as materials for replacing the LiCoO2. Among these materials, research and development of the lithium nickel composite oxides, in which a large capacity battery may be easily achieved due to a high reversible capacity of about 200 mAh/g, have been more actively conducted. However, the LiNiO2 has limitations in that the LiNiO2 has poorer thermal stability than the LiCoO2 and, when an internal short circuit occurs in a charged state due to an external pressure, the positive electrode active material itself is decomposed to cause rupture and ignition of the battery.
Accordingly, a method of substituting a portion of nickel (Ni) with cobalt (Co) or manganese (Mn) has been proposed as a method to improve low thermal stability while maintaining the excellent reversible capacity of the LiNiO2. However, with respect to LiNi1−xCoxO2 (x=0.1 to 0.3) in which a portion of nickel is substituted with cobalt, excellent charge and discharge characteristics and life characteristics are obtained, but thermal stability may be low. Also, with respect to a nickel manganese-based lithium composite metal oxide, in which a portion of Ni is substituted with Mn having excellent thermal stability, and a nickel manganese cobalt-based lithium composite metal oxide (hereinafter, simply referred to as “NMC-based lithium oxide”) in which a portion of Ni is substituted with Mn and Co, output characteristics are low, and there is a concern that metallic elements may be eluted and battery characteristics may be degraded accordingly.
In order to address the above limitations, a lithium transition metal oxide having a concentration gradient of a metal composition has been proposed. This method is performed by synthesizing a core material, coating the outside of the core material with a material having a different composition to prepare a double layer, mixing the double layer with a lithium salt, and then performing a heat treatment. In this method, metal compositions of the core and the outer layer may be differently synthesized during the synthesis, but, since the formation of a continuous concentration gradient of the metal composition in the formed positive electrode active material is not sufficient, an effect of improvement in the output characteristics may be unsatisfactory and reproducibility may be low.
As another method, research to increase the amount of Ni in the NMC-based lithium oxide has been conducted to achieve high energy density in batteries for small cars and batteries for electric power storage. In general, capacity of a positive electrode active material, lifetime, or stability is in a trade-off relationship in which the lifetime and stability of the battery are rapidly reduced if the capacity is increased. Accordingly, a method of using the NMC-based lithium oxide only in limited composition and voltage range, a method of stabilizing a structure to a limited extent by substituting some compositions of the NMC-based lithium oxide with a heterogeneous element, and a method of reducing a surface side reaction through coating have been proposed. However, all of these methods have limitations in fundamentally improving electrochemical and thermal stabilities of the active material, and, since performance degradation is accelerated at a high voltage, there has been a difficulty in achieving the high energy density in the battery.